Purification of in-polymer GlcUA-5-epimerase will be continued, using 5-H3-labeled partially N-sulfated (GlcNAc - GlcUA)n as assay substrate and release of H3 as the assay reaction. Enzyme source will be mast cell mastocytoma soluble fraction. Heparin synthesized from specifically H3-labeled GlcUA precursors will be converted chemically to the component uronic acids and the latter will be degraded carbon by carbon to locate the H3. Shift of label will be used to help determine th mechanisms of the in-polymer 5-epimerization of GlcUA which occurs during heparin biosynthesis.